Debian Admin published a guide about filter spam with Spamassassin and Postfix
"We have already discussed how to install mail server with postfix+Devcot+SASL +Squirrel Mail in this article we will see how to add spam filter for postfix mail server.Apache SpamAssassin is an extensible email filter that is used to identify spam. Once identified, the mail can then be optionally tagged as spam for later filtering. It provides a command line tool to perform filtering, a client-server system to filter large volumes of mail, and Mail::SpamAssassin, a set of Perl modules allowing Apache SpamAssassin to be used in a wide variety of email systems."
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How to filter spam with Spamassassin and Postfix
